A medium-high voltage energy conversion system (such as a power electronic transformer, a solid state transformer and an energy router) is a device that directly transfers electrical energy from a low voltage into a medium-high voltage. A conventional power electronic transformer is shown in FIG. 1. An input end of the power electronic transformer is connected to a low voltage common direct current bus, to control a voltage at a low voltage common direct current bus input port constant. In this case, the power electronic transformer operates in a direct current voltage source mode.
In the direct current voltage source mode, the power electronic transformer is required to have a sufficient power capacity and a high dynamic response speed, so as to provide support for a low-voltage-side device that is connected to the low voltage common direct current bus, such as a load, a new energy device, an energy storage element. An insufficient power capacity or a low dynamic response speed may result in a fluctuation of the direct current voltage. In the case of the large fluctuation, the undervoltage or overvoltage protection mechanism of the low-voltage-side device may be triggered, which can even result in burnout of the low-voltage-side device.
In addition, a conventional power electronic transformer may also be shown in FIG. 2, which operates in a direct circuit current source mode. In the direct circuit current source mode, if a fault occurs in a direct current microgrid or grid (for example, a grid voltage drops or dips), the power electronic transformer cannot effectively provide support for the direct current microgrid or grid.
Therefore, it is desired to provide a control method for a medium-high voltage energy conversion system, such that the power electronic transformer can provide support for the direct current microgrid or grid in a case that a fault occurs in the direct current microgrid or grid.